1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as a resonator or a band-pass filter. More specifically, the present invention relates to a surface acoustic wave device including reflectors arranged in a surface acoustic wave traveling direction on both sides of a region of an IDT electrode, and a duplexer including the surface acoustic wave device.
2. Description of the Related Art
A surface acoustic wave device is widely used as a band-pass filter at an RF stage of a cellular phone.
The surface acoustic wave device includes an IDT electrode on a piezoelectric substrate. A formation region of the IDT electrode is excited with an alternating current applied to the IDT electrode. A space has been arranged in a package of the surface acoustic wave device so as not to impede vibration. This arrangement naturally has caused the surface acoustic wave device to be bulky.
Japanese Unexamined Patent Application Publication No. 2006-217226 described below has proposed a structure that allows this type of surface acoustic wave device to be miniaturized. FIG. 9 is a sectional front view of the surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2006-217226. The surface acoustic wave device 501 includes an IDT electrode 503 on a piezoelectric substrate 502. A protective film 504 is formed to cover the IDT electrode 503. A surrounding wall 505 is formed on the piezoelectric substrate 502. The surrounding wall 505 is made of an insulating material, and surrounds a formation region of the IDT electrode 503. A lid 506 is laminated onto the surrounding wall 505. The lid 506 is made of an insulating material, and closes a space A surrounded by the surrounding wall 505. The space A is thus formed such that the vibration of the IDT electrode 503 is not impeded.
Bumps 507 and 508 electrically connected to the IDT electrode 503 or the like are formed on the top surface of the piezoelectric substrate 502. The bumps 507 and 508 penetrate through the surrounding wall 505 and reach through-holes 506a and 506b and then protrude from the top surface of the lid 506.
As illustrated in FIG. 10, the surface acoustic wave device 501 is surface-mounted on a wiring board 510. Electrode lands 511 and 512 are formed on the top surface of the wiring board 510. The surface acoustic wave device 501 is surface-mounted on the wiring board 510 such that the bumps 507 and 508 are connected to the electrode lands 511 and 512.
The space A is defined by the surrounding wall 505 and the lid 506 in the surface acoustic wave device 501 in order to prevent the vibration of the IDT electrode 503 from being impeded. The use of the bumps 507 and 508 penetrating through the surrounding wall 505 and the lid 506 achieves a compact and surface-mountable design, even with the space A included. After being mounted on the wiring board 510, the surface acoustic wave device 501 is covered with a mold resin.
If a gap is present between the lid 506 and the wiring board 510, a mold resin may intrude into the gap. The lid can be deformed by a pressure of the intruding mold resin, and there is a possibility that the space A is collapsed and that the inner surface of the lid 506 touches the IDT electrode 503.
Since the lid 506 is in contact with the wiring board 510 in the surface acoustic wave device 501, there is no gap, and any fault caused by the pressure of the mold resin is unlikely.
If miniaturization and flat design is further implemented, the lid 506 becomes thinner. If the lid 506 is thinner, the presence of the space A tends to cause the lid 506 to deform during the resin molding. There is a possibility that the lid 506 is directly or indirectly in contact with the IDT electrode 503. If the lid 506 is deformed and the inner surface of the lid 506 is in contact with the region of the IDT electrode 503, filter characteristics and resonance characteristics may be degraded.